Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WO 95/02662 ~' ~ ~ ~ ,~~, PCT/LIS94/07874
-1-
METHOD FOR PURIFYING ALCOHOL ESTERS
Field of the Invention
The present invention relates to the transesterification reaction between
alcohols and triglycerides such as vegetable oils or animal fats to produce
alcohol
esters of the triglycerides.
Background of the Invention
Alcohol esters of triglycerides have been shown to be desirable alternatives
for
petroleum diesel fuel. One method for producing alcohol esters of vegetable
oils
involves a transesterification reaction between a vegetable oil and an
alcohol. The
transesterification reaction is preferably carried out in an excess of the
theoretical,
stoichiometric quantity of alcohol and a catalyst. The products from the
reaction are:
(1) an alcohol ester of vegetable oil; (2) by-product alcohol, such as
glycerin;
(3) unreacted excess alcohol; and (4) residual and spent catalyst. The by-
product
alcohol is insoluble in the alcohol ester. Accordingly, upon completion of the
reaction
in a reactor vessel, the reaction products separate into two phases: an ester
rich phase
and a by-product alcohol rich phase. The unreacted excess alcohol and residual
and
spent catalyst are distributed between the ester and by-product alcohol rich
phases.
An existing process for the production of a methyl ester of rapeseed oil
involves reacting excess methyl alcohol with the rapeseed oil. The reaction
products
are allowed to separate into the ester phase and alcohol phase. The two phases
are
then separated and then the ester rich phase is washed with water to remove
unreacted methyl alcohol, residual catalyst, and spent catalyst. This water
washing
step requires large volumes of water, the handling of which contributes to the
size of
the equipment and the expense of operating such equipment. Also, the wash
water
CA 02164121 1997-10-16
including the extracted methyl alcohol and the residual and spent catalyst is
disposed of without further treatment resulting in a loss of these materials.
The
wasting of these materials is an economic loss and an environmental disposal
problem.
Waste oils and animal fats, for example from meat packing plants,
and food processing plants frying oil waste, are also potential sources of raw
material for the transesterification reaction to produce alcohol esters.
Plentiful
sources of these starting materials exist. The purified alcohol esters can be
used
as fuel alternatives and in other high value end products such as detergent
surfactants, herbicides, pesticides, diluents, sticking agents, or lubricating
additives
for hydraulic and transmission fluids.
In view of the increasing interests in alternative fuel sources and
other uses of purified alcohol esters, there exists a need for a more
economical
and environmentally sound process for producing alcohol esters of
triglycerides.
Summary of the Invention
The present invention provides an economical and environmentally
sound process for producing purified alcohol esters of triglycerides, from
sources
such as vegetable oils, fish oil, animal fat, waste oil/fat mixtures, or
possibly
synthetic sources. The present invention relies upon a by-product alcohol,
such
as glycerin, or a recovery alcohol to purify the alcohol ester to a degree
that
alcohol esters produced in accordance with the present invention do not
require
water washing as prior processes require. In accordance with the present
invention, impurities removed by the by-product alcohol or recovery alcohol
can
subsequently be recovered so that the impurities and the alcohol can be
reused.
-2-
62839-1744
CA 02164121 1997-10-16
- Accordingly, the present invention reduces the economic loss of lost
materials and
the environmental problem of disposing of the wash water.
In one aspect, the present invention is a method for purification of an
alcohol ester of a triglyceride produced by a transesterification reaction
between
an alcohol and the triglyceride, the transesterification reaction occurring in
the
presence of a catalyst, the method comprising the steps: separating a first
phase
including the alcohol ester, unreacted alcohol, and catalyst from a second
phase
including a by-product alcohol, unreacted alcohol and catalyst; treating the
second
phase to separate the by-product alcohol from the unreacted alcohol and
catalyst;
treating the first phase with the separated by-product alcohol to separate the
catalyst from the alcohol ester. The removal of unreacted alcohol and catalyst
and other impurities is complete enough that the alcohol ester does not
require
subsequent water washing. Since, in accordance with preferred embodiments of
this aspect of the present invention, the separated by-product alcohol used to
separate the unreacted alcohol and catalyst from the alcohol ester can be
retreated for reuse, the disposal problems of prior processes described above
are
avoided.
In another aspect the present invention provides a method of
purification of an alcohol ester of a triglyceride produced by a
transesterification
reaction between an alcohol and the triglyceride, the transesterification
reaction
occurring in the presence of a catalyst, the method comprising the steps:
separating a first phase including the alcohol ester, unreacted alcohol, and
catalyst
from a second phase including a by-product alcohol, unreacted alcohol, and
catalyst; treating the first phase with a recovery alcohol to separate the
catalyst
-3-
62839-1744
CA 02164121 1997-10-16
from the alcohol ester.
In another aspect, the present invention relates to a method for the
recovery of by-products from a transesterification reaction between an alcohol
and
a triglyceride in the presence of a catalyst, the method comprising the steps:
separating a first phase including an alcohol ester of the triglyceride, first
phase
unreacted alcohol, and first phase catalyst from a second phase including a by-
product alcohol, second phase unreacted alcohol and second phase catalyst;
treating the first phase with a recovery alcohol to separate the first phase
unreacted alcohol and the first phase catalyst from the alcohol ester and
provide a
mixture of the recovery alcohol, first phase unreacted alcohol, and the first
phase
catalyst; and separating at least one component from the mixture of the
recovery
alcohol, first phase unreacted alcohol, and the first phase catalyst.
The invention also provides a method for the recovery of by-products
from a transesterification reaction between an alcohol and a triglyceride in
the
presence of a catalyst, the method comprising the steps: separating a first
phase
including an alcohol ester of the triglyceride, first phase unreacted alcohol,
and
first phase catalyst from a second phase including a by-product alcohol,
second
phase unreacted alcohol and second phase catalyst; treating the first phase
with a
recovery alcohol to separate the first phase unreacted alcohol and the first
phase
catalyst from the alcohol ester and provide a mixture of the recovery alcohol,
first
phase unreacted alcohol, and the first phase catalyst; and separating at least
one
component from the by-product alcohol, second phase unreacted alcohol and
second phase catalyst that form the second phase.
The mixture of the by-product alcohol, first phase unreacted alcohol
-3a-
62839-1744
.;
CA 02164121 1997-10-16
and first phase catalyst can be combined with the second phase after it is
separated from the first phase but before it is treated to separate the second
phase unreacted alcohol from the by-product alcohol and the second phase
catalyst. Alternatively, the mixture of the by-product alcohol, first phase
unreacted
alcohol and the first phase catalyst is not combined with the second phase as
described above. In either case, the individual streams or the combined
mixture is
then separated into streams of by-product alcohol, first and second phase
unreacted alcohol, and first and second phase catalyst. The by-product
alcohol,
first and second phase unreacted alcohol, and first and second phase catalyst
are
then recovered.
In a preferred embodiment, a portion of the by-product alcohol can
be recycled. Since unspent catalyst is contained in the by-product alcohol,
the
recycle of the by-product alcohol also recycles unspent catalyst, which
reduces the
amount of fresh catalyst required.
The by-product alcohol rich phase also includes mono- or di-
glyceride by-products that are essentially insoluble in the ester product. The
mono- and di-glyceride by-products result from incomplete esterification of
the
triglyceride. In another preferred embodiment, the yield of alcohol ester can
be
increased and partial
-3b-
62839-1744
WO 95/02662 PCT/US94/07874
purification of the by-product alcohol can be achieved by using a secondary
reactor to
convert mono- and di-glycerides by-products into more alcohol ester and by-
product
alcohol. By treating the by-product alcohol with additional excess alcohol
and/or
heating, the mono- and di-glycerides can be converted to alcohol ester and by-
product
alcohol. Usage of a secondary reactor increases the yield of both valuable
products
and reduces the amount of residue that must be disposed of.
Brief Description of the Drawings
The foregoing aspects and many of the attendant advantages of this invention
will become more readily appreciated as the same becomes better understood by
reference to the following detailed description, when taken in conjunction
with the
accompanying drawings, wherein:
FIGURE 1 is a schematic flow chart for a method for producing alcohol esters
of triglycerides in accordance with the present invention;
FIGURE 2 is a detailed schematic drawing of the bench scale system described
in the Example;
FIGURE 3 is a detailed schematic of one extraction stage of Figure 2;
FIGURE 4 is a schematic of the bench scale set up of Figure 2 with the
sampling sites identified;
FIGURE 5 is a graph illustrating the effect of ester to glycerin ratio on
methyl
ester from settler No. 1;
FIGURE 6 is a graph illustrating the effect of ester to glycerin ratio on the
final methyl ester product;
FIGURE 7 is a graph illustrating the effect of ester to glycerin ratio on
glycerin from settler No. 1;
FIGURE 8 is a graph illustrating the effect of ester to glycerin ratio on
glycerin from settler No. 2;
FIGURE 9 is a graph illustrating the concentration profile for a 1:1 glycerin
to
ester ratio;
FIGURE 10 is a graph illustrating the concentration profile for a 1:1.5
glycerin
to ester ratio;
FIGURE 11 is a graph illustrating the concentration profile for a 1:2 glycerin
to ester ratio;
FIGURE 12 is a graph illustrating the effect of glycerin to ester ratio on
ethyl
ester from settler No. 1;
FIGURE 13 is a graph illustrating the effect of glycerin to ester ratio on
final
ethyl ester product;
CA 02164121 2000-03-16
62839-1744
FIGURE 14 is a graph illustrating the effect of
glycerin to ethyl ester ratio on glycerin from settler No. 1;
FIGURE 15 is a graph illustrating the effect of
glycerin to ethyl ester ratio on glycerin from settler No.2 ;
5 FIGURE 16 is a graph illustrating concentration
profile for l:l glycerin to ethyl ester ratio; and
FIGURE 17 is a graph illustrating the concentration
profile for a 1:2 glycerin to ethyl ester ratio.
Detailed Description of the Preferred Embodiment
A more detailed description of the present invention
is presented by reference to the previously cited FIGURES 1-17
which are provided for explanatory purposes only and are not
meant to define or limit the scope of the invention.
Esters undergo reaction with alcohols to give a new
ester and a new alcohol. This reaction is catalyzed by either
acid or base and is called transesterification. Triglycerides,
such as vegetable oils or animal fats, and alcohols can be
transesterified to produce purified esters. In the following
description, the present invention will be described in the
context of transesterification of rapeseed oil with methyl
alcohol or ethyl alcohol to produce purified esters. It should
be understood that the following description refers to the
specific triglyceride, vegetable oil; however, the reference to
the vegetable oil is for clarity only and the present invention
applies to triglycerides from sources other than vegetable oil.
Furthermore, other types of alcohols can be employed to provide
purified esters in accordance with the present invention.
Referring to FIGURE 1, the basic flow of a process
formed in accordance with the present invention is illustrated.
Methyl alcohol from storage tank 20 is mixed with catalyst and
CA 02164121 2000-03-16
62839-1744
5a
introduced into transesterification reactor 22 in an amount in
excess of the theoretical stoichiometric amount. A
triglyceride, such as rapeseed oil, from oil storage tank 3 is
also introduced into transesterification reactor 22 with
mixing. After the reaction has proceeded, a first phase 48
containing raw ester, spent catalyst, unspent catalyst, and
unreacted alcohol is withdrawn from the upper portion of
reactor and delivered to liquid extraction system 26. From the
bottom of transesterification reactor 22 is drawn a second
phase 50 that includes by-product glycerin, unreacted alcohol,
unspent catalyst, and spent catalyst. For purposes of
simplicity, the term "catalyst" as used hereinafter will refer
to residual and spent catalyst unless otherwise indicated. The
term "recovery alcohol" as used hereinafter will refer to an
alcohol, such as glycerin, from a source other than the by-
product alcohol from the transesterification reaction. The
recovery alcohol can be used in treating the first phase to
separate the unreacted alcohol and catalyst from the alcohol
ester. First phase 48 is subjected to a liquid extraction in
liquid extraction system 26 to separate unreacted alcohol and
catalyst from the raw ester. In the illustrated embodiment,
glycerin is used as the solvent to extract the
WO 95/02662 PCT/US94107874
-6_
unreacted alcohol and catalyst from the raw ester of the first phase. Glycerin
is
insoluble in the ester of the first phase yet has a high solubility with
respect to the
unreacted alcohol and catalyst. Accordingly, the glycerin extracts the
unreacted
alcohol and catalyst from the raw ester of the first phase. Glycerin is
preferred as a
solvent for the extraction since it is a natural by-product of the
transesterification
reaction. Second phase 50 and the raff'mate in stream 52 from extraction
system 26
which includes the solvent glycerin, unreacted alcohol and catalyst are
delivered to
glycerin storage tank 28. In the illustrated embodiment second phase 50 in
stream 51
and raffinate in stream 52 are combined before delivery to storage tank 28,
but this is
not required. Streams 51 and 52 can be delivered separately to storage tank 28
or
they can be delivered to separate storage tanks (not shown) for further
processing as
described below. In the illustrated embodiment, feed stream 29 from glycerin
storage
tank 28 is delivered to extractive distillation column 30 where it is
distilled to separate
unreacted alcohol from the glycerin and catalyst. The vapors comprising
unreacted
alcohol from the extractive distillation are condensed in condenser 60 and
delivered
via stream 62 to alcohol storage tank 20. Bottoms from extractive distillation
column 30 in stream 66 are delivered to dewatering column 34 where residual
water is
vaporized. The vaporized water is condensed and collected in condenser 68. The
bottoms from dewatering column 34 are delivered via stream 74 to vacuum
distillation
column 36 where the glycerin is vaporized and separated from the catalyst. The
vaporized glycerin is condensed in condenser 76 and delivered to purified
glycerin
storage tank 32 for holding and subsequent use either as a by-product or as a
solvent
for liquid extraction system 26. The bottoms of vacuum distillation column 36
that
comprise catalyst and residual organic materials can be delivered to heat
recovery
furnace 38 where they are combusted to produce ash and flue gas.
In an alternative embodiment, partial catalyst recycle is accomplished by
recirculating a portion of the glycerin from various locations in the process
described
above with reference to FIGURE 1. For example, recycling a part of stream 51,
52,
or 56 or any other catalyst containing stream will reduce the amount of fresh
catalyst
that is required for the reaction. Another possible source of unspent catalyst
is
stream 81 from the bottom of vacuum distillation unit 36. A significant amount
of
unspent catalyst is contained in the second phase or glycerin phase and,
accordingly,
recycling part of the glycerin also recycles the catalyst, which reduces the
amount of
fresh catalyst required. The volume of recycle will depend upon the specific
system
employed and can be readily determined using conventional chemical engineering
techniques.
WO 95/02662 '~ ~ PCT/US94I07874
_'7_
As can be seen from the flowchart, the present invention preferably uses a
recycle stream of glycerin that is used as a solvent in liquid extraction
system 26
wherein high purity ester is separated from unreacted alcohol and catalyst. A
more
detailed description of the feedstocks and various process streams and
equipment is
provided below.
Vegetable oils, animal fats, fish oils and the like that are useful as
feedstock
for the transesterification reaction include those oils that include
triglycerides which
can be transesterified into alcohol esters. Specific types of vegetable oils
include
rapeseed oil, sunflower oil, safflower oil and the like.
The alcohol feedstock can be selected from organic alcohols that undergo an
alcoholysis substitution exchange reaction with the triglyceride to produce
glycerin
and an alcohol ester. Specific examples of suitable alcohols include methyl
alcohol,
ethyl alcohol, propyl alcohol, butyl alcohol and the like.
The transesterification reaction between the triglyceride and alcohol is
catalyzed by bases such as potassium hydroxide, sodium hydroxide, sodium
methoxide, potassium methoxide, sodium ethoxide and the like. The
transesterification reaction can also be catalyzed by acids such as sulfuric
acid.
The particular and preferred conditions for carrying out the
transesterification
reaction are described in Peterson, C.L., Feldman, M., Korus, R., and Auld,
D.L.
(1991), "Batch Type Transesterification Process for Winter Rape Oils" Annlied
En 'n~ eerin~in Agriculture, 7(6) p. 711-716 and "Process Development of
Rapeseed
Oil Ethyl Ester as a Diesel Fuel Substitute" M.S. Thesis by Narendra Bam,
University
of Idaho, July, 1991. As noted above, in order to obtain yields in excess of
95% and
complete conversion of triglycerides, an excess of the alcohol is employed.
Various
excess alcohol amounts can be used in combination with different catalysts,
catalyst
amounts and reactor temperatures. For example, in accordance with the present
invention, a 100% excess of alcohol based on the theoretical stoichiometric
amount of
alcohol can be used at room temperature with 1% potassium hydroxide. When
excess
amounts of alcohol are used, in accordance with the present invention, phase
separation of the ester phase and alcohol phase occurs after the
transesterification
reaction is completed. If sufficient excess alcohol is not employed, the
addition of
additional alcohol may be required to initiate the phase separation. Requiring
the
addition of alcohol after the transesterification is complete will result in
larger
equipment sizes being required to handle the increased volume.
Yelds and conversions in excess of 99% have been reported in literature;
however, these reported yields are based on the disappearance of the
triglyceride
WO 95/02662 PCT/US94/07874
~~~41.2~. -
rather than the appearance of ester. Applicants have observed that there is a
partial
conversion of the triglycerides to mono- or di-glycerides, accordingly the
reporting of
the literature of conversions of yields in excess of 99% based on the
disappearance of
the triglyceride is not necessarily a clear representation of the amount of
ester that is
produced. In other words, a reported 99% conversion of triglyceride may not
necessarily translate into a yield of alcohol ester that would be equal to
complete
conversion of 99% of the triglyceride to ester.
Continuing to refer to FIGURE 1, the individual unit operations of a method
for producing alcohol esters of vegetable oils in accordance with the present
invention
will be described in more detail. The following detailed description will be
in the
context of the transesterification of rapeseed oil with methyl alcohol
catalyzed by
potassium hydroxide. It should be understood that the description is
applicable to
other vegetable oils or animal fats, alcohols and catalysts.
The transesterification reaction between the alcohol and vegetable oil is
carried out in transesterification reaction vessel 22. To provide sufficient
contact
between the vegetable oil and the alcohol for the transesterification reaction
to
proceed, reaction vessel 22 includes an impeller 44 attached to motor 46.
Motor 46
rotates impeller 44 so that it will agitate the reactor volume with just a
slight amount
of splashing. If necessary, reaction vessel 22 can be provided with a jacket
(not
shown) to heat or cool the contents. If the reaction is carried out in a batch
manner,
separation of the first phase and second phase can be achieved by allowing the
reaction mixture to stand without agitation after the reaction is completed.
In
accordance with the present invention, the phase separation occurs without
further
treatment of the reaction mixture because an excess of alcohol is used. After
the
phase separation has occurred, the respective phases can be removed from
reaction
vessel 22 as described below. In the illustrated embodiment, in order to
remove the
lighter ester rich phase 48 and the heavier by-product alcohol rich phase 50,
reaction
vessel 22 is provided with a lower drain 40 adjacent its bottom and an upper
drain 42
positioned above the interface between ester rich phase 48 and the alcohol
rich
phase 50. Upper drain 42 is used for removal of the first phase or ester rich
phase 48
that generally comprises raw ester, unreacted alcohol, and catalyst. The
second phase
or alcohol rich phase SO generally comprises unreacted alcohol, by-product
glycerin
and catalyst and is removed from the lower drain 40.
Alternatively, though not illustrated, when the reaction is carried out in a
batch
manner, alcohol rich phase can be drained from the bottom of the reaction
vessel into
a storage tank. The ester rich phase can be retained in the reaction vessel
and
WO 95/02662 , PCT/US94/07874
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extracted with glycerin in accordance with the present invention.
Alternatively, after
the alcohol rich phase is drained from the reaction vessel, the ester rich
phase can be
drained from the bottom of the reaction vessel and delivered to the liquid
extraction
system. As noted above, while the transesterification reaction and the
extraction of
S the ester rich phase can be carned out on a batch manner, it is preferred
that a
continuous mixer reactor and a continuous settling system be employed as
described
below with reference to FIGURE 2.
Vegetable oil and alcohol are introduced into the reaction vessel22 from
alcohol storage tank 20 and oil storage tank 24 that can be provided with
pumps or be
gravity feed tanks. The alcohol is introduced in a ratio ranging from about 7%
to
40% by weight based on oil used. This amount provides the needed excess so
that
phase separation will occur after the reaction is complete. Prior to
introduction of
alcohol into reaction vessel 22, catalyst is mixed into the alcohol. The
catalyst is used
in an amount ranging from about 0.1% to 2.0% by weight based on oil used.
Reaction times are dependent upon the temperature of the reaction vessel,
catalyst
type and amount, and excess alcohol used. Reaction times less than 60 minutes
can
readily be selected to obtain over 99% conversion of triglycerides when
potassium
hydroxide is used as catalyst and the reaction is carried out at room
temperature.
Liquid extraction system 26 that separates unreacted alcohol and catalyst from
the raw ester of the first phase can be of any conventional design provided
that
adequate contact between the first phase and the solvent is achieved so that
the
unreacted alcohol and catalyst are separated from the raw ester. In accordance
with
the present invention, the ratio of solvent to the first phase should be in a
range that
provides the desired separation. While solvent to ester ratios can be very
small if
more contact stages are used during the extraction process, there is an
economic
tradeoff between less extraction equipment stages and larger solvent flows.
Solvent
to first phase ratios ranging between about 1:1 to about 1:20 or more can
achieve
suitable separation. Preferably, the solvent to first phase ratio ranges
between about
1:1 to about less than 1:4 so that the number of extraction stages is kept at
an
economically reasonable number. Suitable equipment for liquid extraction
system 26
can be operated batchwise or continuously. For example, in a batch process,
after the
first phase and second phase are separated a quantity of first phase may be
mixed with
a quantity of solvent, e.g., glycerin in agitated vessel, after which the
layers are settled
and separated into extract and raffmate. This operation can be repeated if
more than
one theoretical contact stage is required to achieve the desired extraction.
When the
quantities of ester involved are large and several contacts are needed,
continuous flow
WO 95/02662 PCTIUS94/07874
-10-
~~s4~z1
becomes more economical and is preferred. Representative types of extraction
units
include mixer settlers, vertical towers of various kinds that operate by
gravity flow,
agitated tower extractors, and centrifugal extractors. The particular
configuration and
design of the individual components of a liquid extraction system can be
readily
ascertained using conventional chemical engineering calculations and
techniques. One
specific configuration is described in the example that follows.
As noted above, liquid extraction system 26 provides a ra~nate stream 52
comprising the solvent glycerin, unreacted alcohol and catalyst, and an
extract
stream 54 comprising the raw ester. As noted above, the resulting raw ester is
generally of a purity that does not require the water washing required by
prior
processes.
In the illustrated embodiment, raffinate stream 52 from liquid extraction
system 26 is combined with the alcohol rich phase 50 from transesterification
reactor 22 in stream 51 for further processing to separate out the unreacted
alcohol,
residual water, catalyst, and glycerin. As described above, although not
illustrated, a
portion of streams 51, 52, or 56 can be recycled to reactor 22 for the purpose
of
catalyst recycle. The following describes a specific scheme for separating
these
components; however, it should be understood that other process sequences
could be
used to achieve the separation.
Continuing to refer to FIGURE 1, the combined raffinate stream and alcohol
rich stream 56 is delivered to storage tank 28 for accumulation prior to
subsequent
processing. Unreacted alcohol is separated from the glycerin, catalyst, and
residual
water by extractive distillation. Extractive distillation can be carried out
in an
extractive distillation vessel 30 wherein the unreacted alcohol is vaporized
while the
glycerin and water remain in liquid form and the catalyst remains in solution.
The
vaporized unreacted alcohol is recovered and condensed in condenser 60. A
portion
of the condensed unreacted alcohol is returned to storage tank 20 via stream
62 and
serves as feedstock for transesterification reactor 22. A portion of the
condensed
recovered alcohol is recycled via stream 64 to extractive distillation vessel
30 in order
to maintain the needed equilibrium. The particular conditions of the
extractive
distillation can be determined using conventional chemical engineering
techniques and
theories. The bottoms from extractive distillation vesse130 comprise glycerin,
residual water, and catalyst. The bottoms are delivered via stream 66 to
dewatering
column 34 where the residual water is vaporized to separate it from the
glycerin and
catalyst. The vaporized water is collected and condensed by condenser 68 with
a
portion being returned via stream 70 to dewatering column 34 as reflux and the
WO 95/02662 ~ PCT/US94/07874
-11-
remaining portion being disposed of via stream 72. The operating conditions
for the
dewatering column can be determined using known chemical engineering
techniques.
The bottoms from dewatering column 34 that include glycerin, organic residues,
and
catalyst are delivered to vacuum distillation column 36 via stream 74 where
the
glycerin is vaporized to separate it from the catalyst and organic residue.
The
vaporized glycerin is collected and then condensed in condenser 76. A portion
of the
condensed glycerin is returned to vacuum distillation column 36 as reflux via
stream 78 and the remaining portion in stream 80 provides by-product glycerin
and a
source of solvent for liquid extraction system 26 described above. The
particular
conditions of the vacuum distillation can be determined using conventional
chemical
engineering techniques.
The bottoms from vacuum distillation column 36 are collected and delivered
to heat recovery furnace 38, where they are combusted to produce flue gas and
ash.
The ash contains spent and unspent catalyst residue. The heat generated by the
combustion of the catalyst is recovered for use in other unit operations.
Since the
bottoms from vacuum distillation column 36 include spent catalyst and unspent
catalyst, optionally a portion of the bottoms from vacuum distillation column
36 can
be collected and delivered to reactor vessel 22, thus recycling some of the
unspent
catalyst.
In accordance with the present invention, the use of glycerin in the liquid
extraction provides an extract stream of ester that is greater than about 99%
pure.
Preferably, the ester contains less than about 100 parts per million of
impurities such
as residual spent or unspent catalyst. Ester of this purity generally does not
need to
be washed to remove impurities prior to further use as an alternative fuel. By
circumventing the water washing step required by the prior processes, the
creation of
a waste water stream and the waste of alcohol and unspent catalyst is reduced.
It
should be understood that the use of glycerin as a solvent in the liquid
extraction
process can be designed to produce an ester stream of almost any desired
purity
specification. For example, using amounts of glycerin solvent described above
will
provide an ester that is greater than 99% pure; however, if desired, the
amount of
glycerin solvent used and/or the number of stages used in the extraction
system can be
reduced to produce ester products of lower purity. Conversely, increasing the
amount of glycerin solvent used and increasing the number of stages in the
extraction
system will result in an even higher purity ester product.
As described above, the conversion of triglycerides to alcohol esters in the
transesterification reaction can be less than complete. Accordingly, mono- and
di-
CA 02164121 2000-03-16 '
-12-
glycerides are also by-products of the transesterification reaction. Increased
yield of
ester and supplemental purification of the glycerin can be achieved by
subjecting the
second phase or the combined second phase and raffinate from the liquid
extraction
system to a secondary reaction to convert the mono- and di-glycerides into
more ester
and glycerin. For example, the mono- and di-glycerides in the second phase can
be
convened into more ester by adding excess alcohol to the second phase after it
is
separated from the ester rich first phase. This secondary reaction can be
carried out in
a secondary reactor (not shown). The resulting product can be delivered to the
liquid
extraction where the impurities can be separated from the ester.
Alternatively, the
second phase can be heated after it is separated from the first phase in order
to
convert mono- and di-glycerides into more ester and glycerin. A combination of
adding excess alcohol and heating the resulting mixture will also result in
conversion
of the mono- and di-glycerides into ester and glycerin. Wnen the second phase
is
heated at atmospheric pressure, the maximum temperature of heating will be
limited
by the boiling point of the alcohol being used in the transesterincation
reaction.
Pressurizing the vessel in which the second phase is heated would allow
heating at
higher temperatures and would result in even higher yields of ester and
glycerin from
the mono- and di-glycerides.
The example below provides a specific description of one embodiment for
carrying out the transesterification of a vegetable oil using glycerin as a
solvent in
accordance with the present invention.
ExamQle
A bench scale two stage . mixer-settler was constructed and operated. The
apparatus consisted of raw material supply vessels, a continuous reactor, and
a two
stage mixer-settler extractor. A flow sheet diagram of the process is shown in
FIGURE 2. FIGURE 3 provides a detailed view of a mixer-settler section.
Referring to FIGURES 2 and 3, rapeseed oil and a mixture of potassium
hydroxide catalyst in methyl alcohol were delivered to transesterification
reactor 84
via tubing from gravity feed tanks 86 and 88. Two 250 milliliter beakers
fitted with
hose barbs at the top and bottom served as constant head tanks 90 and 92 to
supply
reactor 84 with a constant flow of raw material. Constant head tanks 90 and 92
were
over supplied with raw material from two 1,000 milliliter beakers fitted with
hose
barbs at the bottom. In this way, the 2~0 milliliter beakers were allowed to
constantly
overflow, thereby keeping the liquid level in each tank and the static head
constant.
The rapeseed oil was transported in 1/4" inner diameter, 3/8" outer diameter
Nalgene 280 ester grade polyurethane tubing. This same tubing was initially
used for
*trade-mark
CA 02164121 2000-03-16
-13-
the alcohol-potassium hydroxide mixture, but the tubing was quickly dissolved
by the
mixture. Accordingly, a 1/4" inner diameter, 3/8" outer diameter Van Waters
and
Ropers, Inc. vinyl tubing performed satisfactorily. The rapeseed oil flow was
measured in an Atheson Gas Products 604 15 centimeter dual ball rotameter
calibrated using a graduated cylinder and stop watch to the oil flow rate. The
alcohol
mixture flow was measured in an Atheson Gas Products 601 15 centimeter dug!
ball
rotameter calibrated to the alcohol mixture flow rate.
Transesterification reactor 84 was a 1,000 milliliter beaker fitted with a
hose
barb at the bottom toward the exit point. The volume in the reactor was
controlle~a b~~
the height of the overflow exit tube 94 and was set at approximately 500
milliliters for
this experiment. The agitation of reactor 84 was accomplished by a 38 millir.
°ter
impeller 96 attached to a 5,000 RP'~i Bodine DC motor 98. The motor RP~is were
controlled by a variac (not shown) and were set to agitate the reactor volume
with
just a slight amount of splashing which was approximately 1,700 RPMs.
The reactor efrluent was gravit;r fed from reactor 84 in 1/4" inner
diameter, 3/8" outer diameter ~.lalgene 230 ester grade polyurethane tubing
and was
injected into a 2,000 milliliter beaker that served as a primary settler 100
fitted ;with
hose barbs at the top and the bottom. The feed from reactor 84 was injected at
the
ester glycerin interface 102 in primary settler 100. Ester phase 104 was then
taken off
the top of settler 100 via stream 105 and overrlowed by gravity to first stage
t 06 of
the extraction mixer-settler 112. The glycerin from primary settler 100 was
taken otT
the bottom and passed t~ a waste container 108 via stream 109. The height of
the
glycerin-ester interface 102 in settler 100 was controlled by the height of
glycerin exit
tube 114 with a siphon brake 116 attached. The tubing used to transport the
ester
and glycerin exiting primary settler 100 was 5/16" inner diameter, 7/ i 6"
outer
diameter Tygon tubing. The reason for using larger tubing was because the 1/4"
hose
barbs could not be attached to the beaker and the larger 5/16" barbs.
Mixer-settler extraction section 112 was composed of two mixer vessels 118
and 120 and two settler vessels 122 and 124 that made a two stage mixer-
settler 112
with counter-current flow. Mixer vessels 118 and 120 were 250 milliliter
beakers
with hose barbs on the top io allow overflow of the mixture to the se;::i;~r.
The
mixtures were agitated with 26 millimeter impellers 126 driven by 1,750 RAM AC
Bodine motors 128. All fluids in the mixer-settler area were tranc~:crted in
the 1/4" inner diameter, 3/8" outer diameter Nalgene 280 ester grade
poi;~urethane
tubing.
* trade-mark
WO 95/02662 PCT/US94/07874
-14-
Settlers 122 and 124 were 1,000 milliliter beakers with hose barbs attached to
the bottom and at approximately the 800 milliliter mark on the beaker. The
effluent
from the respective mixer vessels 118 and 120 was injected approximately at
the ester
glycerin interfaces 110. The level of interfaces 110 was controlled by the
height of
the exiting glycerin tubes 130 with siphon brakes 132 just as .the interface
was
controlled in primary settler 100. Ester from first settler 122 passed to
second
mixer 120 via stream 133 and the glycerin from first settler 122 went to waste
container 108 via stream 135. Second settler 124 produced the final purified
ester in
stream 137 and the glycerin from settler 124 was collected in a surge tank
134.
Fresh glycerin was supplied to the second mixer 120 by a constant head
tank 136 similar to tank 86 supplying reactor 84. The glycerin supply system
was
composed of a 1,000 milliliter beaker 138 feeding a 250 milliliter constant
head
beaker 136 fitted with hose barbs on the top and the bottom. The glycerin from
the
second stage effluent surge tank 134 (which was 250 milliliter beaker fitted
with a
hose barb on the bottom) was pumped with a Masterflex Size 14 pump 140 driven
by
a 1,000 RPM drive. Pump 140 was calibrated as closely as possible to meter the
flow
of glycerin to first mixer 118. The flow of fresh glycerin was controlled by
adjusting
the flow with a tube clamp so that the number of drops flowing from supply
tube 142
in a certain amount of time were equal to the number of drops coming from pump
tube 144. This type of control was necessary because no rotameter could
accurately
measure the glycerin flow due to its viscosity.
A total of five experimental runs were made with the mixer-settler described
above. Three runs were made using methyl ester produced in the continuous
reactor.
The two other runs were made by directly feeding unwashed ethyl ester to the
mixer-settler. On each run the ratio of glycerin to ester was changed in order
to
determine how little glycerin could be used.
Methyl Ester Runs
Three methyl ester runs were made with glycerin to ester ratios
of 1:1, 1:1.5, 1:2. The ester production rates for these runs were 3.5
milliliters per
minute, 5.2 milliliters per minute, and 7 milliliters per minute, respectively
with the
glycerin flow rates remaining constant at approximately 3.5 milliliters per
minute.
Each run was made sufficiently long to account for the dead time in the system
so that
ester produced at the start of the run would have time to pass through the
system
before samples were taken. The experimental run times were 12 hours, 8 hours,
and
6 hours, respectively. The transesterification reaction residence times were
WO 95/02662 PCT/US94/07874
-1 S-
approximately 143 minutes, 96 minutes, and 71 minutes, respectively. All of
the
methyl ester runs were conducted at room temperature of approximately
78°F.
Samples were taken from several places along the mixer-settler section
including the feed to the extractors, the glycerin from each settler, and the
alcohol
ester fuel from each settler. These samples were analyzed for alcohol, ester,
and
glycerin content and most samples had a Plant Macro Elemental Screen (PMES)
run
on them. The PMES measures the phosphorous, potassium, calcium, magnesium,
sulfur, and sodium content of the sample by an inductively coupled plasma
analyzer.
The nomenclature for the sample points is shown in FIGURE 4. The results of
the
sample analysis are given in Table 1. The effects of the different glycerin to
ester
ratios are shown in FIGURES 5-11.
FIGURE 5 shows the effect of changing the ratio of glycerin added to the
incoming ester to the composition of the ester leaving first settler 122. In
all cases all
detectable amounts of methanol were removed in the first stage and the
concentration
of potassium is reduced by a factor of 10. For the 1:2 glycerin to ester
ratio, no
potassium was detected which may have been the result of analysis error,
sampling
error, or a reduction in the potassium concentration in the feed to first
settler 122.
FIGURE 6 is similar to FIGURE 5 except that the ester is from second
settler 124 where the final ester product is being analyzed. Here all
concentrations
have been reduced to below the detectable levels except for methanol being
detected
in the sample with a 1:2 ratio. Since the ester from first settler 122 for
this ratio did
not contain any methanol, this sample probably represents a concentration
resulting
from a temporary fluctuation in methanol concentration in the initial reactor
and may
not be truly representative of the steady state concentration.
FIGURE 7 shows the concentrations of the glycerin from first settler 122.
This FIGURE shows that the glycerin is extracting the methanol and the
potassium
from the ester. It can also be seen that as the amount of glycerin added per
amount of
ester decreases, some carryover of the ester into the glycerin is observed.
FIGURE 8
also supports this observation (note that the glycerin from second settler 124
was not
analyzed for potassium). One possible explanation would be that the settling
time was
not long enough to allow for a complete phase separation. Entrainment effects
may
also be a possible cause because of the extreme viscosity of glycerin.
In FIGURES 9-11, the change in ester concentration from the feed to first
settler 122 to second settler 124 is shown. In all cases a massive reduction
in
methanol and potassium is seen from the feed stock to the ester effluent from
the first
stage and with the second stage making little contribution. These FIGURES
illustrate
WO 95/02662 PCT/US94107874
-16-
the effectiveness of the glycerin extraction carried out in accordance with
the present
invention and suggest that one stage could be eliminated, or the ratio to
glycerin to
ester could be further reduced.
Ethyl Ester Runs
Two ethyl ester runs were made with glycerin to ester ratios of 1:1 and 1:2.
The ester feed rates for these runs were 3.5 milliliters per minute and 7
milliliters per
minute, respectively with the glycerin flow rates remaining constant at
approximately
3.5 milliliters per minute. Each run was made sufficiently long to account for
the dead
time in the system so that ester fed to the first mixer 118 at the start of
the run would
have time to pass through the system before samples were taken. The
experimental
run times were both four hours. All the ethyl ester runs were conducted at
room
temperature of approximately 78°F using unwashed ethyl ester made
previously in a
200-gallon batch test.
Samples were taken from the same key places along the mixer-settler section
as the methyl ester run samples. These samples were also analyzed for alcohol,
ester,
and glycerin content as well as having a PMES run on them. The nomenclature
for
the sample points is analogous to the nomenclature used on the methyl ester
runs with
the exception of using an E to indicate ethyl ester instead of the M for
methyl ester.
The results of the sample analyses are given in Table 2. The effect of the
different
glycerin to ester ratios are shown in FIGURES 12-17.
FIGURE 12 shows the effect of changing the ratio of glycerin added to the
incoming ester to the composition of the ester leaving first settler 122. In
all cases,
nearly all of the detectable ethanol was removed in the first stage and the
concentration of potassium was reduced more than 40 times. For the 1:2
glycerin to
ester ratio slightly larger amounts of ethanol and potassium were detected.
This may
indicate that the lower glycerin to ester ratio is less effective in
extracting the ethanol
and potassium. However, the differences between the 1:1 and the 1:2 samples
were
small enough to be within sampling analysis error. This system was less prone
to
fluctuation than the methyl system because the ester used here was prepared in
advance of the experiment and was of constant composition.
FIGURE 13 is similar to FIGURE 12 except that the fuel from second
settler 124 or the final alcohol ester product is being analyzed. Here all
concentrations have been reduced to below the detectable levels indicating
that a
much lower glycerin to ester ratio could be used, or one stage could be
eliminated.
FIGURE 14 shows the concentrations of the glycerin from first settler 122.
This FIGURE shows that the glycerin is extracting the ethanol and the
potassium
WO 95/02662 ~ PCT/US94107874
-17-
from the ester. It can also be seen that as the amount of glycerin added per
amount of
ester decreases, some carryover of the ester into the glycerin is observed.
FIGURE 15 also supports this observation (note that the glycerin from second
settler
124 was not analyzed for potassium). One possible explanation would be that
the
settling time was not long enough to allow for a complete phase separation.
Entrainment effects may also be a possible cause because of the extreme
viscosity of
glycerin. From previous observations, the samples shown in FIGURE 15 appear to
be
reversed. Since there was some ethanol in the ester from first settler 122 for
the 1:2
ratio, it would be expected that the glycerin from second settler 124 would
contain
some ethanol for the 1:2 ratio but not for the 1:1 ratio. These expectations
are just
the opposite.
FIGURE 16 and FIGURE 17 show the change in ester concentration from the
feed to first settler 122 to second settler 124. In all cases, a massive
reduction in
ethanol and potassium concentration is seen from the feed stock to the ester
effluent
from the first stage with the second stage making little contribution. These
FIGURES
illustrate the effectiveness of the glycerin extraction and suggest that one
stage could
be eliminated, or the ratio of glycerin to ester could be fixrther reduced.
The examples show that the glycerin extraction process carried out in
accordance with the present invention successfully removes excess alcohol and
potassium from both methyl and ethyl esters. The continuous reactor worked
well for
the methyl ester, but obtaining a phase separation was difficult with the
ethyl ester,
and a continuous reactor would preferably be run at much higher percentages of
excess alcohol and a larger settler would be required. The experimental
results
showed that all of the excess alcohol and most of the potassium was removed
from
the ester phase after the first stage even with a glycerin to ester ratio to
1:2. Because
of this a one stage mixer-settler could be used to reduce capital investment,
or a two
stage mixer-settler could be used with a much lower glycerin to ester ratio
which
would reduce the operating cost.
Two phenomena were observed with the ethyl ester. First, the untreated ethyl
ester settled a small amount of glycerin over a period of about three weeks.
In the
same period of time no glycerin was observed to settle out of the ethyl ester
that had
been purified in the mixer-settler. The absence of glycerin settling out of
the ester is
an indication of the high purity of the ethyl ester product produced in
accordance with
the present invention. Providing an ethyl ester product that does not settle
out a small
3~ amount of glycerin is desirable from a materials-handling standpoint and
the
standpoint of a concern for product purity or degradation. Second, the ethyl
ester
WO 95/02662 PCT/US94107874
-18-
purified in the mixer-settler did not emulsify nearly as severely as the
untreated ethyl
ester did. Providing a purified ethyl ester wherein the ethyl ester does not
form a
water emulsion may have advantages in high value added products where water
emulsions are undesirable.
j~64~2~
WO 95/02662 - PCT/US94107874
-19-
Table 1 - Methyl Ester Run Results
Methanol Glycerin M. Ester Potassium Sodium Sulfur
Sample # Mass % Mass % Mass % ~g/g ~.g/g ~g/g
MEFS 1:1 3.36% 0.00% 96.64% 440 12 BDL
MEFS 1:1.52.44% 0.00% 97.56% 270 14 BDL
MEFS 1:2 3.82% 0.00% 96.18% 280 15 BDL
MGFS l:l 46.42% 49.93% 3.66% ---- ---- ----
MGFS 1:1.552.32% 47.68% 0.00% ---- ---- ----
MGFS 1.2 48.57% 51.43% 0.00% 36000 50 60
MES 1 1:1 0.00% 0.00% 100.00% 20 30 34
MES 1 1:1.50.00% 0.00% 100.00% 19 25 BDL
MES 1 1:2 0.00% 0.00% 100.00% BDL 60 40
MGS 1 1:1 1.90% 98.10% 0.00% 260 10 BDL
MGS11:1.5 2.67% 97.33% 0.00% 780 10 5
MGS 1 1:2 2.63% 92.80% 4.57% 440 11 BDL
MGS21:1 0.00% 100.00% 0.00% ---- --- ----
MGS21:1.5 0.00% 95.28% 4.72% ---- ---- ----
MGS21:2 0.00% 82.51% 17.49% ---- ---- ----
MEFP 1:1 0.00% 0.00% 100.00% BDL 13 BDL
MEFP 1:1.50.00% 0.00% 100.00% BDL 44 25
MEFP 1:2 0.74% 0.00% 99.26% BDL 15 BDL
---- Indicates that the sample was not analyzed for that component.
BDL - Below detectable levels
WO 95/02662 s PCT/US94/07874
.~ ~ 412'1
-20-
Table 2 - Ethyl Ester Run Results
Ethanol Glycerin E. Ester Potassium Sodium Sulfur
Sample # Mass % Mass % Mass % ~~g p'~g p'~g
EEFS I:1 3.24% 0.00% 96.76% 925 ~ 42 BDL
EEFS 1:2 3.40% 0.00% 96.60% 970 64 16
EES 1 1:1 0.00% 0.00% 100.00% 20 42 BDL
EES11:2 0.21% 0.00% 99.79% 23 33 BDL
EGS I 1:1 2.73% 94.70% 2.57% 780 9 BDL
EGS11:2 3.38% 93.91% 2.71% 1100 12 31
EGS2 1: ~ 0.65% 98.55% 0.80% ---- ---- ----
I
EGS21:2 0.00% 97.02% 2.98% ---- ---- ----
EEFP 1:1 0.00% 0.00% 100.00% BDL 50 30
EEFP 1:2 0.00% 0.00% 100.00% BDL 45 BDL
--- Indicates that the sample was not analyzed for that component.
BDL - Below detectable levels
While the preferred embodiment of the invention has been illustrated and
described, it will be appreciated that various changes can be made therein
without
departing from the spirit and scope of the invention.
